This invention related to lasers. More particularly it relates to tunable, external cavity lasers and laser systems.
An external cavity laser or ECL, is a generic term for a configuration where the optical feedback path extends beyond the facets or boundaries of an optical gain medium. It consists of an optical gain medium (a semiconductor diode laser), optics for coupling the output of the diode laser into the external cavity, one or more wavelength selective filters, and one or more mirrors for defining an external feedback path, possibly with a piezoelectric translator for fine tuning. The external cavity may also contain additional components such as polarization optics, beamsplitters, prisms, telescopes, etc.
The purpose of the external cavity is to provide optical feedback to the laser to control its output characteristics. Some of the characteristics associated with an ECL are a reduction in threshold lasing current and line width, and longitudinal mode selection, and control of the lasing frequency. Solitary diode lasers, or diode lasers with no external reflectors, usually lase at the modes that are at their center of their gain curves. Only a relatively small amount of tuning may be achieved by varying temperature and injection current of the solitary lasers. They also lase at higher threshold currents than do ECLs, which is due to their relatively lower lasing efficiency resulting from the absence of an additional reflector provided by an external cavity. The lower lasing efficiency is evident in the comparison of the greater threshold injection currents required to cause lasing in the solitary lasers.
Present uses of ECLs include spectroscopy and fiber optic test equipment. ECLs are particularly well suited for high-resolution atomic spectroscopy, possessing output wavelengths which match many transitions of various atomic elements including sodium, rubidium, and uranium.
Widespread use of broadly tunable laser diodes in low to moderate resolution spectroscopy applications requires fast, simple, and highly reliable tuning to arbitrary specified frequencies on demand. This may take the form of smooth, quasi-continuous tuning in wavelength, but for many applications step tuning is adequate. The well known problem is that laser diode chip modes, which have about a 1 cm-1 (30 GHz) mode spacing (Free spectral range) interfere with smooth tuning by an external cavity. Unless superb anti-reflection coatings are used on the output facet of the diode, it is difficult or impossible to tune the laser in between the longitudinal chip modes using the external cavity tuning element, which is typically a diffraction grating. Additional problems arise, at higher resolution, from discrete longitudinal external cavity modes, which may have xcx9c0.05 cm-1 spacing.
Achieving a specified operation frequency involves tuning both the diode chip and the external cavity. The former may be accomplished by varying the chip temperature T (which can easily tune over several modes), the injection current i, or (less commonly) the mechanical stress applied to the device. Tuning the external cavity involves adjusting the grating angle and/or cavity length. For cavity designs using Fabry-Perot etalons as tuning elements, the etalon must also be tuned via gap size or tilt angle.
Expert users commonly use these adjustments to achieve satisfactory laser tunes, with the help of diagnostic instruments and operator skill. Once set, short range tuning can be accomplished by several means. Very short tuning ranges can be accessed by varying the external cavity length via piezo devices or otherwise. Somewhat longer scans, of up to a half a chip mode or so, can be attained via variation of injection current. However, if this current tuning is not coordinated with the external cavity length, the tune will consist of a series of external cavity mode hops.
It is an object of the invention to perform an automated calibration procedure to characterize a standard, off-the-shelf diode laser in an ECL configuration and construct a wavelength tuning model. It is a further object of the invention to utilize the wavelength tuning model in normal operation to automate the ability to tune the system to any desired wavelength within the specification of the diode laser.
The apparatus of the invention includes an external cavity laser system with a semiconductor diode laser, a temperature controller for the diode laser, a control for the laser injection current and a means for wavelength selection in the external cavity. The length of the external cavity may also be controlled.
In order to automate system tuning in normal operation, a wavelength tuning model is developed. In order to develop the tuning model, the diode laser system, including the external cavity components, are characterized by measuring the system output over a desired range of operating parameters. This is accomplished by identifying the range of each operating characteristic: laser temperature, injection current and external cavity wavelength and using these ranges in a calibration procedure by incrementally stepping the system through each possible combination of values. At each step, the values are recorded along with the corresponding system output. The data is then assembled into a table of operating characteristics. The length of the cavity may also be incremented and the system output recorded as an additional set of data in the table.
The table of operating characteristics is then analyzed to obtain values of operating characteristics that result in the diode laser operating at a specific solitary longitudinal mode and frequency. These selected values are then used to operate the system in an autotuning fashion where a desired solitary longitudinal mode and frequency is achieved by setting the system to a selected discrete set of values in the table. The table is updated or refined as needed, for instance to compensate for system drift, by reacquiring all, or some, of the operating characteristics and reanalyzing the data. The selected values are also utilized to cause the system to scan a range of frequencies.
At any time during the operation but particularly during calibration, the system output can be measured using a wavemeter, a plane Fabry-Perot etalon, a spherical Fabry-Perot etalon, a grating polychromator, an acousto-optical tunable filter, a spectrum analyzer or a Lyot-filter spectrometer. The important output characteristics are optical power output and frequency.